Two cheers for Freescale Semiconductor: With its seventh-generation high-voltage (HV7) RF LDMOS technology, it has managed to achieve the RF power amplifier performance required for use in WiMax base stations operating in the 3.5 GHz band. This is another important step forward for WiMax: It appears to be the first time RF Laterally Diffused Metal Oxide Semiconductor (LDMOS) technology from any manufacturer has met these power challenges. Freescale already offers a portfolio of 12V GaAs Pseudomorphic High Electron Mobility Transistor (PHEMT) products. It says it will continue development of high-voltage GaAs PHEMT technology which will result in higher-power GaAs devices for use in WiMax system designs and other applications between 2 GHz and 6 GHz. By adding power transistors in RF LDMOS to its GaAs PHEMT technology offerings, Freescale's RF solutions now support practically any high-power wireless infrastructure application, with LDMOS performance up to 3.8 GHz and GaAs PHEMT performance up to 6 GHz.
Samples of the initial 3.5 GHz LDMOS device are available now. The MRF7S38075H is a 75 Watt P1dB RF transistor capable of 42dBm (16W) average power while meeting WiMax performance requirements over the 3.5 GHz band. Samples of 40W and 10W P1dB 3.5 GHz devices will likely be available in February. Advanced HV7 LDMOS devices complement 12V GaAs PHEMT devices for 3.5 GHz WiMax applications, but the new high-voltage GaAs devices currently under development will operate up to 6 GHz. With an operating voltage above 20V, the GaAs devices will achieve output powers as high as 100W while still keeping within the stringent demands of digitally modulated systems. Samples of Freescale's first high-voltage GaAs PHEMTs are expected to be available in Q3 2006.
BACKGROUND: WiMax systems use a 64 quadrature amplitude modulation (QAM) orthogonal frequency-division multiplexing (OFDM) signal. QAM OFDM signaling thus has advantages, but it also presents distinct challenges to the design of power amplifiers. RF power transistor linearity in back-off is critical not only in its spectral form with mask requirements, but also in its quadrature form with the EVM (Error Vector Magnitude) requirement. Before Freescale's announcement, silicon LDMOS technology did not offer an acceptable level of RF power performance at 3.5 GHz, meaning that compound semiconductor devices, such as GaAs PHEMTs, were the only choice available for designers. Freescale's advanced 3.5 GHz HV7 LDMOS devices now offer the efficiency, linearity, and EVM performance required by WiMax systems, providing designers with a choice between compound semiconductors and silicon LDMOS. As we said above, this is an important step forward for WiMax.